In recent years, wiring density and mounting density on substrates such as printed circuit boards, wafers, and flexible substrates (which may be hereinafter referred to as “mounting substrates”) have been further improved. The mounting substrate has many copper electrodes for soldering electronic components. On the copper electrodes are formed solder bumps, to which the electronic components are soldered in order to be mounted on the mounting substrate.
Solder bumps are in micro-size and uniform in shape, size, and the like. It is required that solder bumps be formed only at necessary portions. As a solder bump formation method satisfying such a requirement, Patent Document 1 has proposed a technique and the like for easily forming minute and uniform bumps by using a screen plate provided with openings for forming paste bumps with a paste and characterized in that it includes a rigid first metal layer, a resin-based adhesive layer, and a second metal layer, and the openings of the adhesive layer and the second layer have a smaller diameter than that of the opening of the first metal layer.
Meanwhile, in an electronic component such as a connector, a QFP (Quad Flat Package), an SOP (Small Outline Package), or a BGA (Ball Grid Array), sizes of connecting terminals, such as lead terminals, can vary. To solder electronic components with connection terminals having varied sizes without any soldering failure, it is necessary to reduce the influence of size variation in the electronic components by increasing the thickness of solder bumps formed on a mounting substrate. When a compact electronic component such as a CSP (Chip Size Package) is among electronic components mounted on a mounting substrate, the size of a solder bump for such a compact electronic component is very small and minute.
As a typical method for forming a solder bump, there is known a method of directly dipping (immersing) a mounting substrate provided with copper electrodes in a molten solder. However, when solder comes into contact with a copper electrode, the copper combines with tin contained in the solder in order to produce a Cu—Sn intermetallic compound. The Cu—Sn intermetallic compound is formed in a manner that the copper electrodes are corroded by the tin in the solder, which is thus sometimes called “copper corrosion”, “copper erosion”, or the like (hereinafter referred to as “copper corrosion”). Such copper corrosion can reduce reliability on copper electrodes as electrical joints and thereby can deteriorate reliability on a mounting substrate. Accordingly, it is necessary to shorten the time for dipping a mounting substrate in a molten solder in order to suppress copper corrosion. Therefore, there has been considered a method (a dipping method) in which a spare solder layer is formed on copper electrodes on a mounting substrate and then the mounting substrate is dipped in a molten solder.